*{
    margin: 0;
    padding: 0;
}
.container{
    width:1000px;
    margin: 0 auto;
}
.col{
    float: left;
    background-color: yellowgreen;
    width: 33.333333%;
    height: 100px;
}
.container::after{
    content: "";
    display: table;
    clear: both;
}

@media (max-width: 800px) {
    .container{
        width: 100%;
    }
    .col{
        width: 100%;
    }
}